This invention relates generally to radio frequency signaling methods used in communication systems. More specifically this invention relates to a multi-level frequency modulated signalling system having at least two data streams and the receiving means for receiving the signal. Although the communication system is described in view of a paging system, it is to be understood that the invention described herein is not limited to paging systems.
Early paging systems used unique tone signals to frequency modulate (FM) or amplitude modulate (AM) a radio frequency carrier. These unique signals were directed to a specific pager wearer to sound an alert and to activate the audio section of his receiver for a voice message which followed. Because of the length of a typical voice messages is 10 seconds or longer a fully loaded radio channel might have the capacity to serve only 1,000 to 3,000 customers. The selective call tone signalling portion of these transmissions required a small percentage of the total channel air time of a typical voice message. Recognizing the need to serve more users per channel a tone only class of service was developed. These tone only signals took very little air time thus paging service could be provided at much lower cost than for voice service. A fully loaded RF channel of tone only paging might service 50,000 to 100,000 users.
Very little information is communicated to the pager wearer in a tone only page ie a first tone sequence would indicate "call home", while if the pager was equipped to receive a second tone sequence it might indicate "call the office". The need for a service with higher channel capacity than voice and higher information content than tone only became apparent.
The more recent development of low power battery operated message displays and changes in the radio frequency modulation methods from tone to digital made it possible to send data messages to pagers in much less channel air time than the equivalent voice message.
The advent of numeric data display paging resulted in tremendous growth in the paging user base. This increased customer demand for data paging service combined with the increased information content of a message received by a pager has created channel congestion especially in the high population density areas of the world. Many paging system owners in these urban areas are being forced to improve existing systems or acquire additional RF channels along with all the accompanying equipment, antenna sites and interconnect equipment required to implement new systems, in order to meet the demand for paging service.
Those system owners that choose to modify existing overcrowded systems must start retiring pagers using slow speed signaling protocols and replace them with pagers operating at higher speed signaling protocols. At present this requires that the available RF channel air time must be shared in a gross way between at least two separate signaling protocols. The newer signaling protocol must appear often enough on the channel to provide a reasonable grade of service while maintaining the older signalling protocols. When the wait time measured from the time a message originates with a telephone call to the time it is received by the paging receiver is too long it tends to be interpreted by the page initiator as a missed page resulting in a second call being entered, thus further congesting the channel. It is quite obvious that this transformation of a paging system would benefit from a less disruptive change. A method which did not require large intervals of time to be removed from the already over crowded channel in order to start the conversion to a higher speed protocol would be a big improvement. A method which overlaid the existing protocol without interfering with the existing protocol would be ideal.
As describe above it is of great importance to the owner of an existing over crowded paging system to acquire the tools for an up grade to higher capacity at reasonable cost with minimum disruption to his existing customer base, and to provide of future expansion using higher data rate signalling protocols.